Ventilator with compliance isolation switch

ABSTRACT

A ventilator apparatus having (1) a main outlet which supplies breathing gas during each inspiratory phase and supplies none during each expiratory phase, (2) a pressure switch that provides gas under pressure during each inspiratory phase and is cut off and bled to atmosphere during each expiratory phase, is connected to a patient&#39;&#39;s breathing circle via a series of elements having a substantial volume and a substantial compliance. The present invention interposes a compliance isolation switch between these elements and the patient&#39;&#39;s breathing circle. This switch has a control means connected by a control line to the pressure switch for opening the switch at the beginning of each inspiratory phase and for closing it at the beginning of each expiratory phase.

United States Patent [191 Ollivier [451 June 28, 1974 VENTILATOR WITH COMPLIANCE ISOLATION SWITCH Primary ExaminerRichard A. Gaudet Assistant ExaminerHenry J. Recla Attorney, Agent, or FirmOwen, Wickersham &

[75] Inventor: Louis A. Ollivier, Menlo Park, Calif.

Erickson [73] Assignee: Veriflo Corporation, Richmond,

Cahf- 57 ABSTRACT [22] Filed: Oct. 1, 1973 A ventilator apparatus having (1) a main outlet which supplies breathing gas during each inspiratory phase [2]] Appl. 402258 and supplies none during each expiratory phase, (2) a Related US. Application Data pressure switch that provides gas under pressure dur- [63] Continuation-impart of Ser. No. 299,148. Oct. 19, mg each p p y Phase is Cut Off and b to 1972, abandoned, atmosphere during each expiratory phase, 18 connected to a patients breathing circle via a series of e1- [52] US. Cl. 128/l45.8 ments having a substantial volume and a substantial [51] Int. Cl A6lm 16/00 compliance. The present invention interposes a com- [58] Field of Search 128/ 145.8, 145.5, 140, pliance isolation switch between these elements and 128/142, 142.2, 142.3, 188 the patients breathing circle. This switch has a control means connected by a control line to the pressure [56] References Cited switch for opening the switch at the beginning of each UNITED STATES PATENTS inspiratory phase and for Closing it at the beginning Of 3,662,751 /1972 Barkalow l28/l45.8 each explratory phase 8 Claims, 3 Drawing Figures SUPPLY I s l8 l9 1% PRESSURE I PRESSURE TIME 4/4 REGULATOR SWITCH ovERRmE I I 47 2 "NEBULIZER" T62 48 '7 l6 6 PRESSURE /2| T 3 SETTING 49 CYCLE FLOW I r J 50 GENERATOR I CONTROLLER 59 H SENSOR I 8 Mm. 43 46 28 OUTLET 1 DETECTOR 1 BACTERIA RELAY 7o-- FILTER I PREssuRE T 32 TIM5ALIELUME l ASSEMBLY I 34 l 13 I ASSIST-CONTROL 72 M I OPERATION SWITCH EXHALATION 75 ,73 3' 40- 3 BREATHING 78 (VOLUME CYCLED) G BREATHING CASCADE 5B CONDUIT I CIRCLE 76 HUMIDIFIER 34 (Pc "Pc2)\ rL to PATIENT PATENTi-IU U I 3820.539

FlG 2 93 FIG. 3

VENTILATOR WITH COMPLIANCE ISOLATION SWITCH CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-partof application Ser. No. 299,148, filed Oct. 19, 1972 and now abandoned.

BACKGROUND OF THE lNVENTION This invention relates to improvements in ventilators for supplying human lungs with air or oxygen. It relates to a particular problem concerned with the compliance of the portions of the ventilator lying beyond the outlet valve, and it relates to a compliance isolation switch for use in this circuit.

Between the main outlet of the ventilator and the patient airway, there are often a number of components, each of which has significant internal volume. For example, there may be a bacteria filter and a humidifier, and there may be large diameter tubes for conducting the air or oxygen. When these components are all taken together, their combined compliance is significant. By the term compliance is meant the volume change per unit of pressure change; when the inspiratory pressure is multiplied by the compliance, the product is the volume of breathing gas at atmospheric pressure that must be-supplied in order to fill these components to the needed pressure, before any of thebrea thing gas is supplied to the patient.

For example, when certain types of ventilators operate in a mode called the control mode," the patient is supplied with a predetermined volume of breathing gas for each cycle of breathing during the inspiratory phase; during this control mode a pressure is created in the patients airway and in-the components which connect the ventilator to the patients airway. A portion of the volume delivered is retained in these components of the breathing circuit in order to develop that pressure. This retained volume is equal to the compliance times the inspiratory pressure. For example, a typical breathing circuit may have a compliance in the range of 3 to 5 ml per cm of water. If the inspiratory pressure reaches 40 cm of water, then the compliance represents a volume of 120 to 200 ml of breathing gas retained by the breathing circuit. Thus, if the volume delivered is preset at 500 ml for a typical adult patient, a significant portion of this amount, amounting to 25-30 percent of the volume supplied by the ventilator, does not go to the patient, but is used to supply the amount needed for compliance, and then that amount is wasted during the expiratory phase by expelling it to the atmosphere. The spirometer which measures the volume during the expiratory phase includes in that measurement the volume which comes out of the breathing circuit and so gives a somewhat false impression of how much the patient is using.

The wasting of gas is, of course, expensive, and the figures obtained from the spirometer, intended to show how much ventilation gas the patient is consuming, are misleading.

However, with infants there are drastic effects. When infants are being provided with their breathing gas by a ventilator, the effect of the compliance on the breathing circuit becomes dramatically significant. Even a low-volume breathing circuit may have a compliance of 2-3 ml per cm of water, and at an inspiratory pressure of 40 cm of water, a volume of -120 ml is retained in the breathing circuit. whereas the infant itself is breathing only about 50 ml per cycle. This means that in order to supply the infant with this 50 ml, the ventilator must be set to deliver a volume between and ml, so that the amount of gas wasted far exceeds the amount of gas consumed.

Thus, with an infant, it becomes difficult to determine the volume that should be delivered by the ventilator in order to provide the infant with the desired volume through the patient airway. It would be very important in this instance to know the compliance of the breathing circuit and to be sure that it does not change significantly with time, such as the change that occurs due to a change in volume of the humidifier as the water level is lowered. These changes can become quite important to a small infant.

Another important factor is that the flow pattern of the inspiratory flow into the lung of any patient is affected by the compliance of the breathing circuit. It is desirable, usually, to begin the inspiratory phase with a large and substantially instant inflow, but when the compliance is large sometimes even larger than the amount of gas being supplied to the lung during a whole breathing cycle the effect is that a large amount of flow is accumulated in the breathing circuit at the beginning of the inspiratory phase, during the time while the pressure builds up in that breathing circuit to an amount sufficient to overcome the resistance of the patients airway. When the patient initiates an inspiratory phase, as when the device is on assist or on assistcontrol, he expects to receive a large flow of breathing gas as soon as he triggers the ventilator, whereas actually he does not get it until somewhat later. In order to trigger the ventilator, the patient makes an inspiratory effort, and withdraws a small volume from the breathing circuit to create a negative pressure. The volume withdrawal is related to the compliance of the breathing circuit. If the compliance is 3 ml per cm of water, it will take a withdrawal of 3 ml to create an inspiratory effort of one centimeter of water.

The present invention seeks to solve these problems by substantially reducing the compliance effect of the circuit between the outlet of the ventilator and the pa tients airways. The purpose is to reduce this compliance effect to a relatively insignificant amount.

SUMMARY OF THE INVENTION The present invention provides a compliance isolation switch which operates on a control signal from the ventilator to open the switch at the beginning of the inspiratory phase quite promptly, and to close it at the end of the inspiratory phase so that it is closed during the entire expiratory phase. By means of the compliance isolation switch, the volume of breathing gas necessary to meet the compliance is retained in the circuit elements between the compliance switch and the outlet, during the expiratory phase. For example, the volume of the humidifier, bacteria filter, and the conduits are filled with gas during the expiratory phase. Then when the patient initiates an inspiratory phase, he gets an instant response.

Thus, this compliance isolation switch, when properly located, overcomes the undesirable conditions set forth in the preceding section of this specification; it does this by isolating the portion of the breathing circuit which contains the components with compliance from the low volume, low-compliance parts which make up the exhalation manifold and the breathing circle.

The compliance isolation switch is placed in the breathing circuit upstream of the exhalation manifold and the breathing circle. Its actuation is synchronized by the ventilator. It is open during the entire inspiratory phase, and it is closed during the entire expiratory phase. As a result, the volume of the breathing circuit upstream of the compliance isolation switch is not exhausted during the expiratory phase, and the pressure therein remains at the value which was reached at the end of the preceding inspiratory phase. Then, when the ventilator is in the control mode to supply a preset volume, it will establish, after a very few initial breathing cycles, a steady condition in which the end inspiratory pressure is determined solely by the compliance of the lung itself and the compliance of the breathing circle with its exhalation manifold.

For example, with the ventilator in the control mode in the first cycle, when the patient takes his very first breath under ventilation conditions, the end inspiratory pressure is determined by the combined compliance of the lung and the compliance of the total breathing circuit. In the second cycle, the isolated upstream portion of the breathing circuit has a reduced compliance, since this portion starts to build up from the pres-- sure level reached in the previous inspiratory phase. Thus, with the same delivered volume, the inspiratory pressure will be higher at the end of the second cycle. In the third cycle the compliance of the isolated upstream portion of the breathing circuit is further reduced, and soon the inspiratory pressure is determined only by the lung and the breathing circle, while the isolated portion of the breathing circuit remains at that pressure level during each expiratory phase.

When a ventilator having the compliance isolation switch of this invention is operated in the assist mode, to deliver a preset pressure, the steady-state condition is established immediately in the first breathing cycle.

As a result of the compliance isolation switch of this invention, the compliance of the breathing circle and the associated exhalation manifold can be made very small, for example, less than one-half ml per cm of water. This minimizes the volume retained by the breathing circle during volume preset operation in the control mode of the ventilator, typically at 20 ml at 40 cm of water inspiratory pressure. This makes it much easier to set the volume to be delivered by the ventilator in order to provide the desired volume in the patients lung.

Furthermore, the inspiratory flow pattern is itself improved, because a high flow is provided at the very start of the inspiratory phase. When the compliance isolation switch opens, the airway is supplied from a source of breathing gas which is at the pressure level that it had at the end of the previous inspiratory period.

The volume withdrawal required to create a negative pressure also is very small when the compliance isolation switch of this invention is used, for it takes only about one-half ml to make an inspiratory effort of 1 cm of water.

Since the compliance effect of the components located upstream of the compliance isolation switch is eliminated, it becomes practical to use large volume components, such as bacteria filters, and cascade humidifiers having considerable volumes, no matter what kind of patient it is being used on. The ability to use a cascade humidifier is significant, since this type of humidifier has the preferred approach to the control of 100 percent relative humidity at body temperature, but without some sort of compliance isolation it would waste a considerable amount of breathing gas.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a diagram of a ventilator circuit in combination with a compliance isolation switch embodying the principles of this invention.

FIG. 2 is a view in elevation of a compliance isolation switch embodying the principles of the invention.

FIG. 3 is a view in section taken along the line 33 in FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows diagrammatically a ventilator representing a ventilator with which this invention can be used. Such a ventilator 10 is shown and described in detail in copending application Ser. No. 97,933 filed Dec. 14, 1970. A supply 11 of breathing gas (air or oxygen or oxygen-enriched air under pressure) sends gas at a predetermined pressure P1 to a flow controller 12, a time-volume valve assembly 13, a pressure switch 14, and a regulator 15, via respective conduits 16, 17, 18, and 19. The regulator 15 reduces the pressure P1 to a lower level P2 and sends gas at that lower pressure to a main operation switch 20, via a conduit 21.

The main operation switch has two modes, a control mode during which the switch 20 makes connections via the solid-line passages 22, 23, and 24, and an assist mode, during which the switch 20 makes connections via the broken-line passages 25, 26, and 27.

With the switch 20 in its control mode, gas from the conduit 21 is sent via a conduit 28 to a cycle generator 30. This cycle generator 30 in turn sends gas during the inspiratory phase only at a pressure PC1 to conduits 31 and 32. The conduit 31 returns to the switch 20 where it is connected, during the control mode" by the passage 23 to a conduit 33. The conduit 33 has two branches 34 and 35, the branch 34 sends the gas at the pressure PCl, during inspiratory phases only, to the flow controller 12. The conduit 32 sends gas at the pressure PCl directly to the time-volume valve assembly, where the pressure PC2 is produced in a conduit 36 and is sent by branch conduits 37 and 38 to the cycle generator 30 and the flow controller respectively. The time-volume valve assembly 13, where the volume delivered to the patient is manually set, along with the time over which that volume is to be delivered, also reduces the pressure P1 to a value P3 and sends that value to the flow controller 12. The flow controller 12 comprises an interrelated group of diaphragms and uses the pressures P1, P3, PCI, and PC2 to produce during inspiratory phases only an outlet flow which is proportional to the difference between PCI and PC2, i.e., the outlet flow from the flow controller 12 equals k(PCl-PC2). This outlet flow is delivered to the patient during inspiratory phases via a breathing gas conduit 40 and a main outlet 41.

Thus, in the control mode," the cycle generator 30 generates from the pressures P2 and PCl a breathing cycle, alternating inspiratory phases with expiratory phases, and supplies the pressure PCl during each inspiratory phase to the flow controller 12. The timevolume valve assembly supplies a pressure PC2 and determines the volume of gas to be delivered by the flow controller and the time over which that volume is to be delivered, and the flow controller supplies the breathing gas for the main outlet 41 at a flow k(PC1-PC2) during each inspiratory phase. The switch passage 24 has no effect on this operation.

When the main operation switch is in the assist mode, the passage 25 leads nowhere. The regulator 15 delivers its gas at pressure P2 via a conduit 42 to a detector relay 43 and via a conduit 44 to a time override 45. Both the detector relay 43 (by a conduit 46) and the time override (by a conduit 47, a pressure setting device 48 and a'kin'dtiit fii are fiiiz'iii to a sensor 50 which senses pressure changes in a conduit 51 that, during use, is connected to the patients airway via an airway outlet 52. The sensor 50 is used to initiate inspiratory phases when the switch 20 is in the assist mode.

The time override is also connected by a conduit 53 to the outlet of a pressure regulator 54, the inlet of which is connected by a conduit 55 and the switch passage 27 to a conduit 56. Both the conduit 56 and the detector relay 43 are connected by a conduit 57 and the switch passage 26 to the conduit 33 and thence by the conduit 34 to the flow controller 12, which, as controlled by the sensor 50, sends breathing gas to the patient via the conduit 40 and the main outlet 41. A gauge 58 is connected by a conduit 59 to the airway conduit 51.

The ventilator 10 also provides a nebulizer outlet 60 and an exhalation valve outlet 61, both controlled by the pressure switch 14, which itself controls transmission of the gas at P1 via the conduit 17 and is controlled by the gas in the conduit 35 in both modes of operation. The nebulizer outlet 60 is connected by a conduit 62 directly to the pressure switch 14, while a restrictor 63 and a bleed 64 affect the flow and pressure of gas supplied to the exhalation valve outlet 61.

Beyond the ventilator l0is not only the patient but such necessary or desirable accessories as the humidifier and filter and exhalation valve. Thus, at the main outlet 41, the breathing gas conduit 40 is connected to a conduit 70, which leads to a bacteria filter 71. From there a conduit 72 leads to a cascade humidifier 73 containing water and consuming that water during use so that the volume occupied by air changes during use. A conduit 74 leads from the humidifier 73 to an airway conduit 75 that is directly connected to the patientat a breathing mask or circle 76. An exhalation valve 77 is connected to the breathing circle by a conduit 78 and is actuated by a conduit 79, which is connected to the exhalation valve outlet 61 of the ventilator 10.

The conduits 70, 72, and 74 all have volume, and so do the. filter 71 and humidifier 73; the compliance of these members is the source of the problems discussed earlier.

The present invention provides a compliance isolation switch 80 inserted in the conduit 74 and connected by ashort conduit 81 to the conduit 75. The switch 80 is actuated by a conduit 82 connected to the nebulizer outlet 60, and there is a restrictor 83 at the inlet of the conduit 84, and a blecd orifice 85 connects the conduit 84 to atmosphere.

The compliance isolation switch 80 is shown in more detail in FIGS. 2 and 3. This switch 80 has a body divided into two chambers 91 and 92. One end of the body 90 is provided with a plug 93 closing the chamber 92, and the other end is closed by a plug 94 having an inlet 95 which is provided with a filter 96 and is connected to the conduit 82. The inlet 95 leads to a reduced size orifice 83 beyond which lies the bleed orifice 85 and an inlet passage 98 leading into a chamber 99, which is enclosed by a diaphragm 100. The diaphragm 100 may be held in place by the plug 94 pressing its rim 101 against thebody 90.

The main body 90 is provided with a bore 102 having an enlarged portion 103, a throat 104, and a portion 105. An inlet fitting 106 leads into the enlarged bore portion 103, and an outlet fitting 107 leads away from the lower bore portion 105. A valve 110 is provided with a stem 111 and a stem head 112. The valve 110 is urged by a spring 113 between the plug 93 and the valve 110 toward a closed position with an O-ring 114 sealing against an insert 115 in the throat 104. The spring 113 also urges the head 112 of the stem 111 against the diaphragm 100.

The chamber 91 is connected by the inlet fitting 106 to the conduit 74 and thence to the upstream portion of the breathing circuit, and the chamber 92 is connected by the outlet fitting 107 to the conduit 81, the airway conduit 75, the breathing circle 76, and the exhalation manifold 77. The spring loaded valve 110 with its O-ring seal 114 controls the communication between the two chambers 91 and 92, and the valve 110 is controlled by the actuating diaphragm 100 and the pressure in the chamber 99 behind it.-

The control signal is generated by the pressurereducing network of the inlet orifice 83 and the bleed orifice 85 and is supplied by the nebulizer outlet 60 of the ventilator 10. During the inspiratory phase, a control pressure is therefore applied to the actuating diaphragm 100 and acts to move the valve 110 downward and to open the passage between the two chambers 91 and 92. During the expiratory phase, the control pres sure in the conduit 82 returns to atmospheric level, and the loading spring 1 13 moves the valve 1 10 to its closed position where the O-ring 114 seals the passage between the two chambers 91 and 92. The spring 113 has a force sufficient to seat the valve 110 against the pressure which has been generated in the breathing circuit during the inspiratory phase, while the pressure downstream is exhausted by the exhalation valve 77 during the expiratory phase. However, if excessive pressure should build up in the inlet chamber 91as it would if the 82 were plugged or severed so that the control signal would not be applied to the control chamber 99, or if the control signal failed to be applied for any reasonthen the valve 110 acts as a relief valve and opens when the pressure in the chamber 91 exceeds a preset value determined by the compression characteristics of the spring 1 13. The downstream fitting 107 and I conduit 8l'are connected to the airway pressure sensor 50 by the conduits 75 and 51, so that the sensor 50 within the ventilator is actuated by the pressure that exists at this point, and the gauge 58 of the ventilator 10 indicates the value of that pressure.

As indicated, the pressure to actuate the compliance isolation switch 80 may be taken from the fitting 60 on the ventilator 10 that is usually used for a nebulizer, no nebulizer being used during this type of operation. The pressure at the fitting 60 is typically from l6 to psi and is reduced to a range of between 4 to 6 psi at the compliance isolation switch 80 by the pressure dividing network built into the connecting plug 94, which comprises the small orifice 83 and the bleed orifice 85.

The breathing gas supply from the flow controller 12 via the conduit 40 and the main ventilator outlet 41 is shut off during each expiratory phase, and the compliance with which this invention is concerned lies beyond this ventilator outlet 41. The switch 80 isolates the conduit 70, the bacteria filter 71, the conduit 72, the eascade humidifier 73, and the conduit 74 from the airway conduit 75 during the expiratory phase. These circuit elements are given by way of example, and there may be others; also, different types of filters or humidifiers may be used, or these may be replaced with other types of devices used to service a patient. In any event, the compliance which is isolated comprises everything from the main ventilator outlet 41 to the chamber 92 of the compliance isolation switch 80 and therefore includes the main chamber 91 of the compliance isolation switch 80 itself. The delivery to the patient then goes on to the exhalation valve 77.

To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spiritand scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

I claim:

1. Ventilator apparatus comprising:

a ventilator having means providing an inspiratory phase and an expiratory phase, a main outlet, means for opening said main outlet for supplying a preset volume of breathing gas through said main outlet during each inspiratory phase and for closing said outlet during each expiratory phase, and a pressure switch for generating a control signal having a positive pressure during each inspiratory phase and cut off and bled to atmosphere during each expiratory phase,

a patients breathing circle having in series an inlet tubing, a tee connection to the patient, a connection tubing, and an exhalation valve which during the patients exhalation opens said breathing circle to the atmosphere,

a main tube connected to said main outlet,

a series of elements connected to said main tube, said series of elements having a substantial volume and a substantial compliance, and

a compliance isolation switch interposed between and connected to said series of elements and said inlet tubing of said patients breathing circle, said compliance isolation switch comprising control means connected to said pressure switch and applying said control signal for opening said compliance isolation switch at the beginning of each said inspiratory phase to provide communication for said breathing gas from said series of elements to said breathing circle and for closing it at the beginning of each said expiratory phase, thereby trapping breathing gas in said main tube and in said series of elements at a pressure equal to the pressure reached in the breathing circle at the end of the inspiratory phase.

2. The ventilator apparatus of claim 1 wherein said series of elements comprises a bacteria filter, a humidifier, and conduits.

3. The ventilator apparatus of claim 1 having means for reducing the pressure of said control signal coming from said pressure switch before application to said control means of said compliance isolation switch, comprising a small inlet orifice connected to said pressure switch and a bleed orifice downstream of said inlet orifice, and passage means connecting said inlet orifice to said control means of said compliance isolation switch.

4. The ventilator apparatus of claim 1 wherein said compliance isolation switch comprises:

a housing providing an inlet chamber having an inlet opening connected to said series of elements and an outlet chamber having an outlet opening connected to said breathing circle, with a throat between them,

valve means for opening and closing said throat,

spring means urging said valve toward its closed position, and I a diaphragm in said housing in engagement with said valve means and providing a control chamber on the opposite side of said diaphragm from said valve means, said diaphragm and control chamber comprising said control means,

said control chamber being connected to said pressure switch so that said control signal places increased pressure on said diaphragm during each inspiratory phase and causes said diaphragm to open said valve means and enable passage of gas between said inlet chamber and said outlet chamber, and removes the pressure from said control chamber during each expiratory phase, so that spring means then closes said valve means and isolates said inlet chamber from said outlet chamber and thereby isolates said series of elements from said breathing circle.

5. The ventilator of claim 4 having means presetting said valve means and said spring means to open said valve means even without application of said control signal if the pressure in said inlet chamber should exceeda predetermined value.

6. The ventilator apparatus of claim 1 wherein said compliance isolation switch comprises:

a housing providing an inlet connected to said series of elements and therethrough to said main outlet and an outlet connected to said patients breathing circle,

valve means for joining and separating said inlet and outlet, normally urged toward its separating position, and

pressure-responsive means in said housing being in engagement with said valve means and providing a control chamber, said pressure-responsive means and said control chamber comprising said control means,

said control chamber being connected to said pressure switch and having means for (l) placing pressure on said pressure-responsive means during each inspiratory phase and causing said pressureresponsive means to move said valve to its joining position enabling passage of gas between said inlet and said outlet, and (2) removing pressure from pressure-reducing network including a small inlet orifice and a bleed orifice downstream of the inlet orifice.

8. The ventilator of claim 6 having means for opening said valve means also when pressure in said inlet exceeds a predetermined excess value. 

1. Ventilator apparatus comprising: a ventilator having means providing an inspiratory phase and an expiratory phase, a main outlet, means for opening said main outlet for supplying a preset volume of breathing gas through said main outlet during each inspiratory phase and for closing said outlet during each expiratory phase, and a pressure switch for generating a control signal having a positive pressure during each inspiratory phase and cut Off and bled to atmosphere during each expiratory phase, a patient''s breathing circle having in series an inlet tubing, a tee connection to the patient, a connection tubing, and an exhalation valve which during the patient''s exhalation opens said breathing circle to the atmosphere, a main tube connected to said main outlet, a series of elements connected to said main tube, said series of elements having a substantial volume and a substantial compliance, and a compliance isolation switch interposed between and connected to said series of elements and said inlet tubing of said patient''s breathing circle, said compliance isolation switch comprising control means connected to said pressure switch and applying said control signal for opening said compliance isolation switch at the beginning of each said inspiratory phase to provide communication for said breathing gas from said series of elements to said breathing circle and for closing it at the beginning of each said expiratory phase, thereby trapping breathing gas in said main tube and in said series of elements at a pressure equal to the pressure reached in the breathing circle at the end of the inspiratory phase.
 2. The ventilator apparatus of claim 1 wherein said series of elements comprises a bacteria filter, a humidifier, and conduits.
 3. The ventilator apparatus of claim 1 having means for reducing the pressure of said control signal coming from said pressure switch before application to said control means of said compliance isolation switch, comprising a small inlet orifice connected to said pressure switch and a bleed orifice downstream of said inlet orifice, and passage means connecting said inlet orifice to said control means of said compliance isolation switch.
 4. The ventilator apparatus of claim 1 wherein said compliance isolation switch comprises: a housing providing an inlet chamber having an inlet opening connected to said series of elements and an outlet chamber having an outlet opening connected to said breathing circle, with a throat between them, valve means for opening and closing said throat, spring means urging said valve toward its closed position, and a diaphragm in said housing in engagement with said valve means and providing a control chamber on the opposite side of said diaphragm from said valve means, said diaphragm and control chamber comprising said control means, said control chamber being connected to said pressure switch so that said control signal places increased pressure on said diaphragm during each inspiratory phase and causes said diaphragm to open said valve means and enable passage of gas between said inlet chamber and said outlet chamber, and removes the pressure from said control chamber during each expiratory phase, so that spring means then closes said valve means and isolates said inlet chamber from said outlet chamber and thereby isolates said series of elements from said breathing circle.
 5. The ventilator of claim 4 having means presetting said valve means and said spring means to open said valve means even without application of said control signal if the pressure in said inlet chamber should exceed a predetermined value.
 6. The ventilator apparatus of claim 1 wherein said compliance isolation switch comprises: a housing providing an inlet connected to said series of elements and therethrough to said main outlet and an outlet connected to said patient''s breathing circle, valve means for joining and separating said inlet and outlet, normally urged toward its separating position, and pressure-responsive means in said housing being in engagement with said valve means and providing a control chamber, said pressure-responsive means and said control chamber comprising said control means, said control chamber being connected to said pressure switch and having means for (1) placing pressure on said pressure-responsive means during each inspiratory phase and causing said pressure-responsive means to move Said valve to its joining position enabling passage of gas between said inlet and said outlet, and (2) removing pressure from said control chamber during each expiratory phase, so that the valve means then separates said inlet from said outlet.
 7. The ventilator apparatus of claim 6 wherein said control chamber is connected to said pressure switch through a pressure-reducing network lying between said pressure switch and said control chamber, said pressure-reducing network including a small inlet orifice and a bleed orifice downstream of the inlet orifice.
 8. The ventilator of claim 6 having means for opening said valve means also when pressure in said inlet exceeds a predetermined excess value. 